The present invention relates to divalent silicon compounds which are known as silylenes. More particularly, it relates to the use of these compounds to catalyze olefin polymerization reactions.
In recent years there have been efforts directed towards the isolation of compounds containing divalent silicon centers, particularly heterocyclic amido variants. See M. Haaf et al., 120 J. Am. Chem. Soc. 12714-12719 (1998) (synthesis of silylenes); J. Lehmann et al., 18 Organomet. 1862-1872 (1999) (study of silylenes); and U.S. Pat. No. 5,728,856 (synthesis of silylenes). The disclosures of these publications, and of all other publications referred to herein, are incorporated by reference as if fully set forth herein.
While several syntheses of these compounds have been proposed, these syntheses were often inefficient. A need still exists for ways to improve their efficiency, particularly with respect to the synthesis of heterocyclic silylenes.
In any event, these compounds were originally developed in order to provide a new class of reactive components that could be combined with other materials in varied synthesis reactions. It is now desired to find still other uses for them.
Synthesis of olefin polymers from monomers typically requires the use of a catalyst to assist in the polymerization reaction. These catalysts are often inefficient, complex, and expensive (e.g. a mixture of an organotitanium compound with an organoaluminum compound).
Other known techniques for polymerizing olefins involve a free radical process which typically results in highly branched polymers having soft properties. This can be a disadvantage when harder polymers are desired. Thus, a need still also exists for providing improved techniques for the production of olefin polymers.
In one aspect the invention provides methods for producing a polymer. One polymerizes monomers selected from the group consisting of terminal alkene monomers and terminal alkyne monomers in the presence of a catalyst selected from the group consisting of silylenes.
The catalyst is preferably a cyclic (preferably heterocyclic) silylene, such as where the catalyst contains a [Nxe2x80x94Sixe2x80x94N] moiety, with these three atoms being part of an at least partially unsaturated heterocyclic ring of at least five and no more than ten atoms.
In an especially preferred form, the catalyst is: 
In another aspect the invention provides polymers produced by the above methods.
In yet another aspect, the invention provides an improved method for forming a compound having a structure selected from the group consisting of: 
wherein M is Si and wherein R1, R2, R3, and R4, and if applicable R5 and R6, are individually selected from the group consisting of H and alkyl with less than 10 carbons. The method involves reacting a precursor (xe2x80x9cPrecursorxe2x80x9d) selected from the group consisting of: 
with elemental potassium. With respect to the Precursor, M is also Si, and R1, R2, R3, and R4, and if applicable R5 and R6, are also individually selected from the group consisting of H and alkyl with less than 10 carbons. The elemental potassium is added to the reaction at above 2.1 and less than 3.0 (preferably between 2.2 and 2.4) molar equivalents of the Precursor present in the reaction.
The preferred compound formed by this improved synthesis is: 
xe2x80x9cTerminal alkene monomerxe2x80x9d includes any polymerizable alkene monomers having a double bonded carbon at an end of the molecule, and mixtures thereof, including without limitation ethylene, propylene, 1-hexene, alkyl vinyl ethers such as ethyl vinyl ether, styrene, butadienes such as dimethyl butadiene, acrylonitrile, vinyl halides such as vinyl chloride, isobutene, and isoprene. As indicated by the inclusion of ethers, carbon and hydrogen need not be the only elements in the monomer.
xe2x80x9cTerminal alkyne monomerxe2x80x9d includes any polymerizable alkyne monomers with a triple bonded carbon at an end of the molecule, and mixtures thereof, including without limitation acetylene, phenyl acetylene, and other alkyl and aryl acetylenes. Again, carbon and hydrogen need not be the only elements in the monomer.
Preferred silylenes are compounds having a divalent silicon linked on each side to nitrogen such as: [(R1R2R3C)R7N]xe2x80x94Sixe2x80x94[NR8(CR4R5R6)], in which R1, R2, R3, R4, R5, R6, R7 and R8are the same or different and each represents a hydrogen or halogen atom or an alkyl, aryl, alkoxy, aryloxy, amido or heteroaryl residue, and R7 and R8 can also represent the residue xe2x80x94CR1R2R3 or xe2x80x94CR4R5R6, and R7 and R8 can jointly form with the respective adjacent nitrogen atoms and the central silicon atom in an unsaturated heterocyclic ring with at least five ring atoms.
Where the catalyst has a heterocyclic ringed structure, it is preferred that any carbons other than those in the ring which includes the Si not exceed twenty carbons, and preferably not exceed six carbons. For example, the nitrogens can both be linked to tertiary butyl groups.
It has surprisingly been learned that silylenes can be efficient olefin polymerization catalysts, even at relatively low concentrations. Further, these compounds hold out the possibility of creating variants which will provide more control over other polymer attributes such as stereochemistry and cross-linking.
These catalysts are particularly useful to create homopolymers. However, they should also be useful in creating copolymers.
It has also been surprisingly learned that too high levels of elemental potassium in the synthesis reaction (e.g. 3.0 molar equivalent or above) can cause significant ring degradation and purification problems. On the other hand, too low an elemental potassium level in the reaction (e.g. 2.1 molar equivalent or below) can lead to undesirably long required reaction times (e.g. sometimes days are required for the reaction). Thus, a narrow range of molar equivalency is highly preferred.
Advantages of the present invention include providing:
(a) methods of the above kind for catalyzing the production of polymers;
(b) polymers of the above kind which are produced by these methods; and
(c) methods of the above kind for more efficiently synthesizing such catalysts.
These and still other advantages of the present invention will be apparent from the description which follows. The description is merely of the preferred embodiments. The claims should therefore be looked to in order to understand the full scope of the invention.
Most of the examples discussed below used the following xe2x80x9cCatalyst Ixe2x80x9d as the catalyst: 
One possible means of synthesizing this catalyst is described in M. Denk et al., 116 J. Am. Chem. Soc. 2691-2692 (1994).
However, we prefer to modify the last step of this synthesis (the reaction of the dihalide) as follows. In our experiment we provide a three-neck 1000 mL flask equipped with a stir bar, reflux condenser and two stoppers was charged with 19.36 g (72.4 mmol) of a precursor (where Mxe2x95x90Si, R1 and R2xe2x95x90H, and R3 and R4=t-butyl). 350 mL of THF was added to the dichloride precursor to dissolve the compound and yield an approximately 0.2 M solution. This was stirred vigorously, while 6.51 g of elemental potassium (166.60 mmol, 2.3 molar equivalents) was cut into small chunks.
The potassium was rinsed with hexane to remove mineral oil and then added to the dichloride solution all at once under a heavy flow of argon. Once the potassium had been added, the solution was set to reflux for three hours. The reaction was monitored by 1H NMR, and upon complete conversion to the silylene (3 hours), the reaction was stopped, cooled, and filtered through a medium-porosity frit to remove potassium chloride. Upon filtration the solvent was removed in vacuo to yield dark red solid. This solid was sublimed at 90xc2x0 C. and 30 mtorr to yield 9.95 g (70.0%) of the pale yellow silylene.